Prospects for an ionospheric index of neutral thermospheric composition, with space-weather applications

Abstract

We address the need for an easily observable indicator of thermospheric state, and particularly one sensitive to the neutral concentration ratio, [O]/[N2]. Accepting foF2 as the observable of choice, several options for a derived thermospheric daily index are considered; these include midday and nighttime mean values of foF22, and others based on the rate of change of foF22 during sunrise, which might serve as indicators of [O]/[N2]. We show the global morphology of a promising sunrise index over more than three solar cycles, derived from data in the National Geophysical Data Center Ionospheric Digital Database (NGDC/IDD) and similar databases. There are well-defined dependencies on solar activity, season, geography, and storm-disturbance level. We compare the indices to ESRO4 satellite global observations of [O]/[N2] and find correlation coefficients as low as 0.3 (southern winter) and as high as 0.8 (close passes, all data), depending upon hemisphere and season, satellite/ground-site proximity, satellite altitude, etc. Using median foF22 to compare our sunrise index to the Mass Spectrometer-Incoherent Scatter (MSIS-90) model atmosphere at a constant pressure level, we find similar latitude variations in three longitude sectors and two solar cycle extremes. A successful index of [O]/[N2], available daily from monitoring ionosondes (and perhaps from Global Positioning System/Total Electron Content (GPS/TEC) monitors, derived similarly), would have important applications in aeronomy, space weather, and telecommunications forecasting. Copyright 2001 by the American Geophysical Union.